1. Field of the Invention
The present invention relates to a ferroelectric memory device and a method of manufacturing the same.
2. Description of the Related Art
A ferroelectric memory device comprising a ferroelectric capacitor is widely used to various applications since the ferroelectric memory device is non-volatile and is able to rewrite content being stored. To further extend the applications, it is essential to miniaturize a cell size so that a memory capacity increases.
For the purpose of miniaturizing a cell size, there is known a ferroelectric memory device having a so-called vertical ferroelectric capacitor in which a pair of capacitor electrodes are disposed in vertical facing each other and disposed in a channel length direction of a transistor putting a ferroelectric film in-between (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-289797).
In a conventional horizontal capacitor in which electrodes of the capacitor are disposed in parallel with a silicon substrate, a capacitor size is miniaturized as a cell size is shrunk. However, even though a cell is miniaturized, a parasitic capacitance of a bit line is not so reduced as the reduction ratio of the capacitor. Accordingly, as the cell is more miniaturized, the parasitic capacitance of the bit line is increased more in comparison with a capacitance of the ferroelectric capacitor.
In a vertical capacitor as the subject of the present invention, an area of the capacitor is determined by height of the capacitor. Accordingly, even if the lateral size of a device is miniaturized by shrinking its cell size, a capacitance can be maintained to be balanced with a parasitic capacitance of a bit line.
A voltage necessary to reverse a polarizing direction of a ferroelectric film is determined depending on a ferroelectric material and its thickness. Accordingly, to reduce the voltage necessary to reverse the polarizing direction of the vertical capacitor, a thickness of the ferroelectric film must be reduced. In a conventional processing technology, since a distance between electrodes of the ferroelectric film is determined by capability of a lithography technology being used, the limit of miniaturization is determined by resolution limit of the lithography. Accordingly, it is not easy to further reduce the distance between the electrodes of the vertical capacitor, and thus it is also not easy to reduce the voltage for reversing the polarizing direction of the ferroelectric film of the ferroelectric capacitor.